The present invention relates to coating apparatuses and methods of applying coatings.
Coatings are utilized in a variety of settings to provide a variety of benefits. For example, modern gas turbine engines can include thermal barrier coatings (TBCs), environmental coatings, etc. to help promote efficient and reliable operation. Application of coatings can involve a variety of different application methods, such as plasma-based physical vapor deposition (PVD). When TBCs are applied to gas turbine engine components, such as blades and vanes, using plasma-based-PVD, the components being coated are rotated within a process chamber while a plasma stream directs the coating material at the components. Examples of such known coating processes are disclosed in U.S. Pat. No. 7,482,035 and in U.S. Pat. App. Pub. Nos. 2007/0259173A1 and 2008/0226837A1.
A significant problem with known plasma-based PVD processes is the loss of work-piece temperature to down-stream portions of the plasma-based PVD equipment. A plasma gun generating the plasma stream is the only source of heat in the system. The walls of the process chamber are typically cooled to approximately 15-20° C., and thereby remove heat from the process chamber, and a downstream end of the process chamber includes equipment to collect and cool excess coating material, thereby also removing thermal energy from the process chamber. Components being coated tend to be cyclically heated and cooled as they rotate because portions of the components that face downstream and away from the plasma stream cool to lower temperatures. TBCs are sensitive to thermal conditions during the coating application process, and undesirable thermal conditions can cause detrimental changes to the microstructure of the TBC. In particular, the TBC develops striations and a cauliflower-like structure due to poor temperature control. While in a typical application it is desired to maintain the components being coated at a temperature of approximately 1038° C. (1900° F.), temperatures can range from approximately 871-1093° C. (1600-2000° F.). Moreover, as portions of the components being coated are rotated back to face the plasma stream, separation between new, hot layers of the coating and the cooler interface of previously-applied coating material can make the resultant coating undesirably friable and prone to separation between layers of the coating. These microstructural characteristics are known to cause a debit in service life for the turbine engine component.
One approach known in the art for providing temperature control involves passive thermal shielding. However, passive thermal shielding mitigates only off-axis heat loss to a relatively cold process chamber. The core reason for this effect is the flow-through nature of the coating vapor stream created via the plasma stream. Laterally-oriented passive spray shielding is incapable of ensuring heat-loss from down-stream areas of the process chamber where the plasma plume and waste ceramic vapor are cooled and collected for extraction from the process chamber.
Thus, it is desired to provide a coating apparatus and method with improved thermal stabilization.